1. Field of the Invention
The invention relates to a discharge reactor and uses thereof. Known generic discharge reactors for silent discharges and similar processes have, between two electrodes, at least one dielectric and a gas-containing discharge gap and are connected, usually in a plurality and in parallel, to an alternating voltage source. During each halfwave, microdischarges are ignited in the gap, breakdowns limited by the dielectric, which generate free radicals and thereby trigger specific chemical reactions in the gas.
2. Discussion of Background
General information on silent discharges and their uses may be obtained, for example, from the following publications: U. Kogelschatz: "Silent Discharges and their Applications" in: "Proceedings of the Tenth International Conference on Gas Discharges and their Applications", Vol. II, Swansea 1992 and B. Eliasson, U. Kogelschatz: "Nonequilibrium Volume Plasma Chemical Processing", IEEE Transactions on Plasma Science 19/6 page 1063-1077 (1991).
Discharge reactors have long been used to produce ozone from O.sub.2 or air for the treatment of drinking water and other purposes. In addition to the abovementioned publications, reference is made, in this respect, to U. Kogelschatz, B. Eliasson: "Ozone Generation and Applications" in J.-S. Chang, A. J. Kelly, J. M. Crowley: "Handbook of Electrostatic Processes", Marcel Dekker, Inc. (1995), and to DE-C-32 20 018.
Another application, which is very useful in light of the increasingly urgent reduction in the emission of greenhouse gases, is the reaction of CO.sub.2 and H.sub.2 in methanol and water, see A. Bill, A. Wokaun, B. Eliasson, E. Killer, U. Kogelschatz: "Greenhouse Gas Chemistry", Energy Convers. Mgmt. 38, Suppl., page 415-422 (1997), and J. U. Holtje: "Untersuchung der Makrokinetic der heterogen katalysierten Synthese aus Kohlendioxid und Wasserstoff zu Methanol", ["Investigation of the macrokinetics of the heterogeneously catalyzed synthesis of carbon dioxide and hydrogen into methanol"], dissertation, Rheinisch-Westfalische Technische Hochschule, [Technical University of Rhine-Westphalia], Aachen 1991. The joint reaction of the greenhouse gases CO.sub.2 and CH.sub.4 into synthesis gas or syngas, a mixture of CO and H.sub.2, is also important in this connection.
Other applications are the decomposition of pollution gases, for example in smoke gases from garbage incineration plants, but also in the exhaust gases of automobiles (see DE-C-195 18 970), and excimer lamps delivering UV radiation which is in a narrow frequency band and which is generated during the decay of excited states of inert gas atoms (see, for example, EP-B-0 547,366).
Various types of discharge reactors are known. Thus, the electrodes may be designed, for example, as parallel plates, or as concentric tubes. Without exception, an appropriately shaped dielectric is used, which separates the electrodes continuously and which consists of at least one layer of solid nonconductive material, for example glass. Said layer may be arranged directly on an electrode or else be spaced from the two electrodes. It is also possible to arrange two layers of this type so as preferably to adjoin the electrodes in each case. In the space between the electrodes there is, in each case, at least one discharge gap, into which the gaseous educts of the desired chemical reaction are introduced and in which microdischarges are formed under the influence of the electric field built up between the electrodes, said microdischarges producing highly reactive intermediate products, namely free electrons and radicals, of which the reactions with one another and, above all, with gas molecules or gas atoms result in the desired products with a yield which depends on various boundary conditions.
The supply voltage applied between the electrodes may correspond to the power supply frequency, as in early ozone generating plants, but, in modern plants, the frequency is usually substantially higher with a view to as high a yield as possible and may enter the GHz range.
The power consumption P of the gas during the silent discharge conforms to the law EQU P=4fC.sub.D U.sub.B (U-(1-.beta.)U.sub.B), (1)
f being the frequency of the supply voltage, U its amplitude, C.sub.D the capacitance of the dielectric, U.sub.B the mean drop voltage of the microdischarges, and EQU .beta.=C.sub.S /C.sub.D (2)
being the quotient from the capacitance C.sub.S of the discharge gap and the capacitance C.sub.D of the dielectric.
Thus, in the case of fixed values for the frequency f, the amplitude U and the capacitance of the dielectric C.sub.D, power consumption depends on U.sub.B and .beta. which, in turn, depend on the gap width of the discharge gap d. In the case of the boundary conditions which are otherwise given (gas composition, pressure and temperature in the discharge gap), the power consumption P and, together with it, the yield of the discharge reactor can therefore be optimized by adjusting this variable.
In actual fact, however, the optimal width of the discharge gap is usually so small that, in the case of discharge reactors which are sufficiently high-performance for economic production and correspondingly large, production tolerances place limits on the gap setting and the actual gap width is to a greater or lesser extent above the optimum.
It is known, admittedly, to fill the discharge gap with a material which occupies part of the volume and leaves free an interconnected branched gas volume. Thus, heaps of particles consisting, for example, of ceramic, said particles filling the discharge gap, are described for generic or similar reactors in JP-A-103 903/89, JP-A-038 881/96, JP-A-261 034/89 and U.S. Pat. No. 5,254,231. In heaps of this kind, however, cavities of widely varying and hardly controllable size occur and the porosity as a whole is low, so that the volume is utilized poorly and flow resistance for the gas flowing through is high. The granulates are also difficult to handle and their properties may easily be impaired by mechanical actions.
DE-A-42 20 865 discloses a generic discharge reactor, in which the discharge gap is filled with glass wool, quartz wool or mineral wool. However, these materials likewise leave free cavities of varying extent which is difficult to control. The discharge gap can be filled up completely only with difficulty on account of their mechanical properties. The same publication also mentions the possibility of applying a porous layer to one of the electrodes. However, this is merely a relatively thin layer consisting of a catalyst material or of a carrier material for the latter, said material taking up only a small part of the discharge gap.